Electronic trip circuit breakers are often interconnected with control circuitry to coordinate the timing of their opening. Control circuitry having a low impedance common or circuit ground conductor creates a partial electrical path between the two circuit breakers. The National Electrical Code (NEC) requires grounding of the secondary windings of instrument current transformers, such as those employed on the neutral conductor of circuit breakers with integral ground fault protection. These grounded current transformers can complete the current paths between the two breakers and cause an objectionable flow of current from one trip unit to the other. If a first circuit breaker sees a ground fault, the combination of two circuit breakers having grounded current transformers and trip units tied together by a low impedance common conductor can cause the ground fault signal to flow between the first breaker and the second breaker. The first breaker will see a ground fault that is lower than the actual fault while the second breaker will sense a ground fault when none exists. It is necessary to prevent this flow of current between the two breakers so that ground fault conditions will be accurately sensed.
In previous designs, the secondary windings of the phase current transformers and neutral current transformers were isolated from the trip unit circuitry by secondary current transformers, which were used to increase the effective current transformer ratio. In designs where a moderate current transformer ratio is acceptable, secondary current transformers are an unnecessary expense and are not utilized. In these designs another method of stopping the objectionable flow of current must be devised. One design uses optocoupler in the control circuits which isolate the control circuits of different circuit breakers. Another method of isolating the ground fault signals is described herein.